Shallow Foundations - Building Construction

Most shallow foundations are simple concrete  footings. A  column footing is  a square block of concrete, with or  without steel reinforcing, that accepts  the concentrated load placed on it  from above by a building column and spreads this load across an area of soil large enough that the allowable bearing stress of the soil is not exceeded.

A wall footing or  strip footing is a continuous strip of concrete that serves the same function for a loadbearing wall (Figures 2.30 and 2.31).

A column footing and a wall footing of concrete. The steel reinforcing bars have  been omitted from this illustration for clarity.
Figure 2.30 A column footing and a wall footing of
concrete. The steel reinforcing bars have  been omitted from this illustration for
clarity.
These concrete foundation walls for an apartment building, with their steel formwork not yet stripped, rest on wall footings. For more extensive illustrations of wall and  column footings
Figure 2.31 These concrete foundation walls for an apartment building, with their steel formwork
not yet stripped, rest on wall footings. For more extensive illustrations of wall and  column footings
To minimize settlement, footings are usually placed on undisturbed soil.  Under some circumstances, footings
may be constructed over engineered fill,  which is earth that has been deposited under the supervision of a soils engineer. The engineer, working from the results of laboratory compaction tests on samples taken from the soil used for filling, makes sure that the soil is deposited in thin layers at a controlled moisture content and compacted in accordance with detailed procedures that ensure a known loadbearing ca-pacity and long-term stability.

Footings appear in many forms in different foundation systems. In  climates with little or no ground frost, a concrete  slab on grade with thickened edges is the least expensive foundation and floor system that one can use and is applicable to one- and two-story buildings of any type of construction (see Chapter 14 for further
information on slabs on grade). Or, in colder regions the edges of a slab on grade may be supported with deeper wall footings that bear on soil below the frost line. For floors raised above the ground, either over a  crawlspaceor a  basement, support is provided by concrete or masonry foundation walls supported on concrete strip footings (Figure 2.32). When building on slopes, it is often necessary to step the footings to maintain the re-quired depth of footing at all points around the building (Figure 2.33). If soil conditions or earthquake precau-tions require it, column footings on steep slopes may be linked together with reinforced concrete  tie beams to avoid possible differential slippage between footings.


 Three types of substructures with shallow foundations. The slab on grade  is the most economical under many circumstances, especially where the  water table lies near the surface of the ground. A crawlspace is often used under  a floor structure of wood or steel, and gives much better access to underfloor  piping and wiring than a slab on grade. Basements provide usable space for  building occupants.
Figure 2.32 Three types of substructures with
shallow foundations. The slab on grade  is the most economical under many
circumstances, especially where the  water table lies near the surface of the
ground. A crawlspace is often used under  a floor structure of wood or steel, and
gives much better access to underfloor  piping and wiring than a slab on grade.
Basements provide usable space for  building occupants.
Foundations on sloping sites, viewed in a cross section   through the building. The broken line indicates the  outline of the superstructure. Wall footings are stepped to  maintain the necessary distance between the bottom of the  footing and the surface of the ground. Separate column  foundations, whether caissons, as shown here, or column  footings, are often connected with reinforced concrete  tie beams to reduce differential movement between the  columns. A grade beam differs from a tie beam by being  reinforced to distribute the continuous load from a bearing  wall to separate foundations.
Figure 2.33 Foundations on sloping sites, viewed in a cross section
 through the building. The broken line indicates the
outline of the superstructure. Wall footings are stepped to
maintain the necessary distance between the bottom of the
footing and the surface of the ground. Separate column
foundations, whether caissons, as shown here, or column
footings, are often connected with reinforced concrete
tie beams to reduce differential movement between the
columns. A grade beam differs from a tie beam by being
reinforced to distribute the continuous load from a bearing
wall to separate foundations.
Footings cannot legally extend beyond a property line, even for a building built tightly against it. If the outer
toe of the footing were simply cut off at the property line, the footing would not be symmetrically loaded by the column or wall and would tend to rotate and fail. Combined footings and cantilever footings solve this problem by tying the footings for the outside row of columns to those of the next row in such a way that any rotational tendency is neutralized (Figure 2.34).
Either a combined footing (top) or a  cantilevered footing (bottom) is used  when columns must abut a property  line. By combining the foundation for  the column against the property line,  at the left, with the foundation for the  next interior column to the right in a  single structural unit, a balanced footing  design can be achieved. The concrete  reinforcing steel has been omitted from  these drawings for the sake of clarity.
Figure 2.34 Either a combined footing (top) or a
cantilevered footing (bottom) is used
when columns must abut a property
line. By combining the foundation for
the column against the property line,
at the left, with the foundation for the
next interior column to the right in a
single structural unit, a balanced footing
design can be achieved. The concrete
reinforcing steel has been omitted from
these drawings for the sake of clarity.
In situations where the allowable bearing capacity of the soil is low in relation to the weight of the build-
ing, column footings may become large enough that it is more economical to merge them into a single mat or  raft foundation that supports the entire building. Mats for very tall buildings may be 6 feet (1.8 m) thick
or more and are heavily reinforced (Figure 2.35).

Pouring a large foundation mat. Six truck-mounted pumps receive concrete from a  continuous procession of transit-mix concrete trucks and deliver this concrete to the  heavily reinforced mat. Concrete placement continues nonstop around the clock until  the mat is finished to avoid “cold joints,” weakened planes between hardened concrete  and fresh concrete. The soil around this excavation is supported with a sitecast concrete  slurry wall. Most of the slurry wall is tied back, but a set of rakers is visible at the lower  right.
Figure 2.35 Pouring a large foundation mat. Six truck-mounted pumps receive concrete from a
continuous procession of transit-mix concrete trucks and deliver this concrete to the
heavily reinforced mat. Concrete placement continues nonstop around the clock until
the mat is finished to avoid “cold joints,” weakened planes between hardened concrete
and fresh concrete. The soil around this excavation is supported with a sitecast concrete
slurry wall. Most of the slurry wall is tied back, but a set of rakers is visible at the lower
right.
Where the bearing capacity of the soil is low and settlement must be carefully controlled, a  floating foundation is sometimes used. A floating foundation is similar to a mat foundation, but is placed beneath a building at a depth such that the weight of the soil removed from the excavation is equal to the weight of the building above. One story of ex-cavated soil weighs about the same as five to eight stories of superstructure, depending on the density of the soil and the construction of the building  (Figure 2.36).
A cross section through a building with  a floating foundation. The building  weighs approximately the same as the  soil excavated for the substructure, so  the stress in the soil beneath the building  is the same after construction as it was  before.
Figure 2.36 A cross section through a building with
a floating foundation. The building
weighs approximately the same as the
soil excavated for the substructure, so
the stress in the soil beneath the building
is the same after construction as it was
before.

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